Bring a large pan of water to the boil and boil the sweet potato fries for 5 minutes, drain and set aside. Preheat the oven to 200°C (fan oven) and put a large roasting tray with a couple of tablespoons of oil in the oven to heat up. When the oil is hot, place the sweet potato fries on to the tray with the mixed herbs and a sprinkling and salt and pepper and return to the oven. Cook for 30-40 minutes until crispy and browned.
While the fries are cooking heat a litre of vegetable oil to 190°C in a large heavy based pan. For the fish, mix the yeast, sugar and sparkling water together in a large bowl. Add the flour, rum and vinegar with a sprinkling of salt and pepper. Leave to the side until it starts to bubble. Dip the cod fillets into the batter and make sure they're fully coated then place in the hot oil and fry for 3-4 minutes until browned then remove with a metal slotted spoon and place on a plate lined with kitchen paper to drain. Sprinkle with salt.
Make the onion rings. Mix together the flour, sparkling water and jerk seasoning then season generously. Coat the onion rings in the batter and fry in batches for a few minutes until browned. Places the cooked onion rings on a paper towel lined plate and sprinkle with salt.
While the fish and onion rings are frying make the curry sauce. Melt the butter in a saucepan over medium heat and add the onion and scotch bonnet. Stir through the flour and curry powder followed by the coconut milk and chicken stock. Bring to a simmer and cook for 5-10 minutes. Finish by stirring through the coriander and lemon juice.